This invention relates to apparatus for determining the genuineness of paper currency and, in particular, to apparatus for determining the genuineness of United States one-dollar bills.
Devices that perform one or more tests on U.S. one-dollar bills or other paper currency to determine their genuineness prior to registering credit in a change-making or in a dispensing operation are well known in the art. Typically in such devices, the paper currency being validated is moved along a path along which various optical, magnetic or edge-sensing tests are performed on the bill. On failing any of these tests, the bill is returned to the user, and no credit is given.
Many of these devices exploit the fact that certain areas of genuine U.S. bills are printed with ink containing magnetizable material, while certain other areas are printed with ordinary nonmagnetic ink. Thus, in the apparatus disclosed in U.S. Pat. No. 3,485,358, issued to D. E. Hooker, the portrait area of a one-dollar bill is first magnetized and then moved past a magnetic head which senses the parallel vertical lines in the background portion of the portrait. The signal from the head is fed to a tuned amplifier, the output of which reaches a certain critical magnitude if the lines on the bill being tested are actually printed with magnetic ink and are spaced from one another by the proper distance. Failure of the amplifier output to reach the critical magnitude causes the bill to be rejected.
In another such apparatus, described in U.S. Pat. No. 3,966,047, issued to L. E. Steiner, the magnetic head is also used to scan a portion of the bill, such as the portions containing the Federal Reserve seal or serial number, printed with nonmagnetic ink. If a magnetic signal is detected at this point, as would happen if the inserted bill were a photocopy made with magnetic toner, the bill is rejected as being a counterfeit.
Devices such as described in the above-identified patents are subject to several types of error. In the device shown in the Hooker patent, the magnetic signal is linearly amplified by a tuned amplifier and then fed to a detector to provide an envelope signal which is sampled at the proper time. The amplitude of the sampled envelope signal thus depends not only on the spacing and magnetic content of the lines being scanned, but also on such extraneous factors as the gain of the amplifier, the amount of dirt that has accumulated on the magnetic head, and the degree to which the magnetic ink has been worn off the bill from normal handling. Slight deviations in the resonant frequency of the LC tuning circuit will also effect the amplitude of the envelope signal, particularly where a high-Q circuit is employed.
All of these extraneous factors taken together result in a region of uncertainty in which the level of the envelope is not necessarily indicative of either a genuine or a counterfeit bill. If the threshold envelope level for acceptance is simply set high enough to ensure against false acceptances of counterfeit bills, there will also necessarily be many false rejections of genuine bills, resulting in customer dissatisfaction as well as decreased revenue.